Iridium NHC based catalysts for transfer hydrogenation processes using glycerol as solvent and hydrogen donor

Article abstract of DOI:10.1021/om200796c

A series of iridium and ruthenium N-heterocyclic carbene based catalysts of general formula [IrI₂(AcO)(bis-NHC)] or [Ru(η⁶-arene) (NHC)CO₃] have been tested in the reduction of several organic carbonyl compounds using glycerol as solvent and hydrogen donor, by the transfer hydrogenation methodology. The Ir(III) complexes with a chelating bis-NHC ligand and sulfonate groups were the most efficient, due to their solubility in the reaction media and to the strong electron-donor properties of the bis-carbene ligands. The same two catalysts were moderately active in the reduction of olefins and alkynes and, more remarkably, show excellent chemoselectivity in the reduction of the alkenic double bond of α,β-unsaturated ketones, a valuable process for which glycerol had never been used before.

Full text of DOI:10.1021/om200796c

ARTICLE
pubs.acs.org/Organometallics
Iridium NHC Based Catalysts for Transfer Hydrogenation Processes
Using Glycerol as Solvent and Hydrogen Donor
Arturo Azua, Jose A. Mata,* and Eduardo Peris*
Departamento de Química Inorgꢀanica y Orgꢀanica, Universitat Jaume I, 12071 Castellꢀon, Spain
ABSTRACT: A series of iridium and ruthenium N-heterocyclic carbene based catalysts
of general formula [IrI2(AcO)(bis-NHC)] or [Ru(η6-arene)(NHC)CO3] have been
tested in the reduction of several organic carbonyl compounds using glycerol as solvent
and hydrogen donor, by the transfer hydrogenation methodology. The Ir(III) com-
plexes with a chelating bis-NHC ligand and sulfonate groups were the most efficient,
due to their solubility in the reaction media and to the strong electron-donor properties
of the bis-carbene ligands. The same two catalysts were moderately active in the
reduction of olefins and alkynes and, more remarkably, show excellent chemoselectivity
in the reduction of the alkenic double bond of α,β-unsaturated ketones, a valuable
process for which glycerol had never been used before.
’ INTRODUCTION
the development of greener chemical processes.17 Glycerol can
be envisaged as a suitable hydrogen source for transfer hydro-
genation processes to ketones and imines if a suitable catalyst
is used. The hydrogen transfer is expected to selectively occur
from the secondary alcohol, yielding dihydroxyacetone (DHA)
as oxidation product. To our knowledge, only two reports
regarding transfer hydrogenation of glycerol have recently
appeared.18,19
The rise of green chemistry has drawn attention to synthetic
procedures that promote atom economy and the use of nontoxic
reagents.1 Well-known cases of green catalysis include hydrogen
transfer methodologies for the reduction of ketones or imines
and oxidation of alcohols and amines, for which the hydrogen
donor (typically 2-propanol) or acceptor (e.g., acetone) are easy-
to-handle, environmentally friendly reagents.2À7
We have recently been interested in the design of N-hetero-
cyclic carbene based catalysts for “hydrogen-borrowing” processes.20
In our research, we also focused our attention on the preparation
of highly effective catalysts that may be capable of reducing inert
molecules such as carbon dioxide, using 2-propanol as the
hydrogen source.21À23 For these reactions, we found that the
best catalysts were those combining high stability, strong elec-
tron-donor ligands, and high solubility in polar solvents. These
three properties were achieved by using chelate abnormally
bound NHC ligands with sulfonate functionalities.21 These
properties of the catalyst, which were valid for the use of an
inert hydrogen acceptor, should also be valid for the use of an
inert hydrogen donor and thus may be applied to the utilization
of glycerol as the hydrogen source in a transfer hydrogenation
process. In this context, we now report the use of a series of
iridium(III) catalysts for the transfer hydrogenation of double
bonds using glycerol as hydrogen donor.
The choice of a green solvent is among the 12 principles of
green chemistry.1 A green solvent must meet numerous criteria,
such as low toxicity, nonflammability, nonmutagenicity, nonvo-
latility, and widespread availability. Although many solvents meet
the aforementioned criteria, glycerol has recently caught the
attention of many researchers, due to its unique physical and
chemical properties, its extraordinarily low cost, and itsready
availability.8 Glycerol is the main coproduct of the vegetable oil
industry, and its use has been boosted by the rapid emergence of
biodiesel in the market.9,10 As a solvent, glycerol has a number of
advantages, such as its ability to dissolve inorganic salts, acids,
bases, metal complexes, and organic compounds that are poorly
miscible in water. Its high boiling point (290 °C) allows reactions
to be carried out at high temperatures and makes distillation of
the reaction products a feasible separation technique. Addition-
ally, glycerol is a nontoxic biodegradable and nonflammable
solvent for which no special handling or storage precautions
are needed.8 Apart from its use as a solvent, new and innovative
catalytic processes based on the use of glycerol have been
recently developed.11À16 In this context, the development of novel,
selective chemistry to provide new applications for glycerol-
derived products remains a key challenge.
’ RESULTS AND DISCUSSION
Compounds 1À4 of general formula [IrI2(AcO)(bis-NHC)]
or [Ru(η6-arene)(NHC)CO3] (Scheme 1) were prepared
Hydrogen-borrowing activation of alcohols may be consid-
ered as an important contribution of organometallic catalysts to
Received: August 25, 2011
Published: September 30, 2011
r
2011 American Chemical Society
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Scheme 1
as previously described by Crabtree24 (complex 3) and us
(complexes 1, 2, and 4).21À23,25 Similar complexes are known to
be very efficient in transfer hydrogenation processes. Complexes
1, 2, and 4 incorporate sulfonate functionalities that make them
insoluble in most organic solvents and very soluble in polar
solvents such as MeOH and water. The solubility is reversed in
the complex [IrI2(AcO)(bis-nBu-NHC)] (3) due to the absence
of the sulfonate group, being very soluble in nonpolar solvents
such us chloroform and dichloromethane. We recently described
that the ruthenium complex 4 [Ru(η6-arene)(NHC)CO3] is
highly active in the isomerization of allylic alcohols in water
(TON 900) and could be recycled up to eight times without loss
of activity.25 Complexes 1 and 2 are active in the reduction of
carbon dioxide to potassium formate under transfer hydrogena-
tion conditions with a maximum TON of 2700 in water as
solvent.21 Complex 3 has been used for comparative reasons.
It is a well-known transfer hydrogenation catalyst highly active
in the reduction of ketones and is very stable toward air and
moisture.24
In the catalytic transfer hydrogenation, unsaturated substrates
are reduced using a hydrogen source that consists of an easily
oxidizable molecule such us isopropyl alcohol. We have devel-
oped very active catalysts for this reaction based on rhodium,26
iridium, and ruthenium.20 Our initial approach using glycerol as
hydrogen donor was carried out using acetophenone as substrate.
A first set of experiments was done to optimize the reaction
conditions. In a typical experiment acetophenone, base, catalyst,
glycerol, and a cosolvent were heated to 80À120 °C for the
appropriate time (Table 1). Under these reaction conditions
acetophenone is partially reduced to 1-phenylethanol, thus
validating our initial objective. Under the same reaction condi-
tions, the catalysts [IrI2(AcO)(bis-NHC)] (1) and [IrI2(AcO)-
(bis-a-NHC)] (2; bis-a-NHC = bis abnormal NHC) redun-
dantly afforded the best catalytic performances (entries 1À4).
Catalyst 3 [Ru(η6-arene)(NHC)CO3] showed low activity,
most probably due to its lower solubility, and the Ru-based
catalyst 4 shows negligible activity (entries 5À8). The better
performances of catalysts 1 and 2 compared to that of 3 may be
attributed to their higher solubility in polar solvents, such as
glycerol. Catalyst 2 is the most efficient due to the presence of the
abnormally bound bis-NHC ligand, which has a strong electron-
donor capacity and may favor the catalytic process. The same
tendency was previously observed in the reduction of CO2 under
transfer hydrogenation conditions.21 Longer reaction times of
Table 1. Optimization of Reaction Conditions in the Re-
duction of Acetophenonea
entry
cat.
solventb
T (°C)
t (h)
yield (%)b,c,d
1
1
1
2
2
3
3
4
4
1
1
2
2
2
1
2
3
1
2
glycerol
glycerol
glycerol
glycerol
glycerol
glycerol
glycerol
glycerol
H2O
80
120
80
15
24
15
24
15
20
15
24
20
24
20
24
48
20
20
20
20
20
40
45
68
69
18
22
2
2
3
4
120
80
5
6
120
80
7
8
120
100
120
100
120
120
120
120
80
10
45
50
53
71
89
12
16
3
9
10
11
12
13
14
15
16
17
18
H2O
H2O
H2O
H2O
CH3CN
CH3CN
toluene
DMSO
DMSO
120
50
120
85
a Reactions were carried out with 0.5 mmol of acetophenone, and
0.5 mmol of KOH. b 0.8 mL of glycerol/0.8 mL of solvent. c 0.5 mmol
of anisole was used as standard. d Yields determined by GC based
on 1-phenylethanol produced.
14À24 h or a temperature increase from 80 to 120 °C does not
produce a considerable improvement in the yield of 1-phenylethanol.
We observed that the addition of a cosolvent highly influenced
the catalytic outcomes (Table 1, entries 9À18). For instance, the
addition of water slightly improve the catalytic outcomes (entries
9À13), while the addition of a moderately coordinating solvent
such as MeCN practically stops the catalytic activity (entries
14 and 15). Using toluene as cosolvent. a two-phase system
was produced. In this case only traces of 1-phenylethanol
were observed. The best catalytic results were obtained when
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Table 2. Reduction of Acetophenone with 1,2-Propanediola
Table 3. Reduction of Aromatic and Aliphatic Carbonyl
Derivatives with Glycerola
entry
cat.
solventb
t (h)
yield (%)c,d
1
2
3
4
5
1
1
2
2
2
1,2-propanediol
DMSO
14
14
14
14
20
64
52
70
60
80
1,2-propanediol
DMSO
DMSO
a Reactions carried out with 0.5 mmol of acetophenone and 0.5 mmol
of KOH at 120 °C. b 0.8 mL of 1,2-propanediol/0.8 mL of solvent.
c 0.5 mmol of anisole was used as standard. d Yields determined by GC
based on 1-phenylethanol produced.
DMSO was used as a cosolvent, especially when the catalyst
[IrI2(AcO)(bis-a-NHC)] (2) was used.
It is important to point out that our optimized results (Table 1,
entries 3, 4, and 12) are significantly better than those previously
reported by Crotti and co-workers, in which a series of
[Ir(diene)(NÀN)X] (diene =1,5-hexadiene, NÀN = bipyridine,
X = Cl, I) catalysts were used, and maximum conversions of
acetophenone of 26% were achieved under similar reaction
conditions.18
Because the selective deoxygenation of 1,2-propanediol to
give n-propanol has been recently described,27 we thought that
this might be a good candidate as the hydrogen source in transfer
hydrogenation reactions. As observed in Table 2, 1,2-propane-
diol is an effective hydrogen donor in the reduction of aceto-
phenone. The addition of a cosolvent did not significantly
improve the yield to the final product. As was observed in the
case of glycerol, the best catalytic outcomes were observed for the
catalyst [IrI2(AcO)(bis-a-NHC)] (2).
a Reactions were carried out with 0.5 mmol of substrate and 0.5 mmol of
KOH at 120 °C.
Table 4. Reduction of Olefins and Acetylenes Using
Glycerola
In order to check the versatility of our catalysts, we decided
to study the substrate scope of 1 and 2 with other carbonyl
derivatives. Table 3 shows the results of the reduction of several
aromatic and aliphatic ketones and aldehydes (benzaldehyde),
using a 2.5 mol % loading of catalyst in a 1:1 mixture of glycerol
and DMSO at 120 °C. While complexes [IrI2(AcO)(bis-NHC)]
(1) and 2 show similar activity in the reduction of 2-naphthyl
methyl ketone (entries 1À4), the complex [IrI2(AcO)(bis-a-
NHC)] (2) was a significantly better catalyst for the reduction of
benzophenone, for which a maximum yield of 91% (entry 7) was
obtained in the production of the final alcohol. Aliphatic ketones
were reduced to a much lesser extent (entries 8À13).
entry
substrate
cat.
product
styrene
T (°C) t (h) yield (%)b,c,d
1
2
phenylacetylene
1
2
1
2
1
1
2
1
1
1
2
120
120
120
120
70
14
14
14
14
14
14
14
14
14
20
3
30
12
18e
3e
3
4
5
17
25
9
6
styrene
ethylbenzene 120
7
120
70
8
16
35
38
55
9
1,5-cycloctadiene
cyclooctene
120
120
120
Remarkably, the reduction of benzaldehyde to benzyl alcohol
is quantitatively produced by catalyst 2 in only 1.5 h (entry 15),
thus exemplifying the high activity of this catalyst toward the
reduction of aromatic aldehydes. The activity of 2 toward this
substrate is substantially higher than that shown by 1 (compare
entries 14 and 15). The activity of 2 in the reduction of
benzaldehyde in glycerol is a clear improvement over previous
results reported by Wolfson and co-workers, where full conver-
sion to benzyl alcohol was obtained under similar reaction
conditions after 24 h, using [RuCl2(p-cymene)]2 as catalyst.19
As seen from the results shown in Table 4, both 1 and 2 were
poorly efficient catalysts in the reduction of olefins and acet-
ylenes. The reduction of phenylacetylene yields styrene, 30%
10
11
a Reactions were carried out with 0.5 mmol of substrate and 0.5 mmol of
KOH at 120 °C. b 0.8 mL of glycerol + 0.8 mL of DMSO. c 0.5 mmol of
anisole was used as standard. d Yields determined by GC based on
product formation. e 0.5 mmol of sodium acetate used instead of KOH.
being the maximum yield achieved by catalyst 1 (entries 1À5).
The reduction of styrene to ethylbenzene is achieved with a
maximum yield of 25%, again provided by the catalyst [IrI2(AcO)-
(bis-NHC)] (1) (entries 6À8). Internal olefins such as 1,5-cyclooc-
tadiene can be transformed into cyclooctene in 55% yield when
catalyst 2 [IrI2(AcO)(bis-a-NHC)] is used (entries 9À11).
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Table 5. Reduction of Benzylidene Ketone and Dibenzylidene Ketone with Glycerola
a Reactions carried out with a 2.5 mol % catalyst loading, 0.5 mmol of substrate, KOH (0.5 mmol) at 120 °C in 1.6 mL of glycerol. b 0.5 mmol of anisole
was used as standard. c Yields determined by GC based on product formation. d Reactions carried out at 100 °C.
Allyl ketones (or α,β-insaturated ketones), can be considered
as activated olefins. The selective hydrogenation of this type of
substrate has attracted great attention because of its wide
application in the synthesis of pharmaceutical compounds.28À30
Only a few catalysts have recently shown good chemoselectivities
in the reduction of the double bonds of this type of enone,31À34
although this reaction has never been carried out with glycerol as
the hydrogen source. Due to the catalytic properties shown by 1
and 2 in the reduction of olefins and aliphatic ketones, we
thought that we could have a good chance to study their selectivity
in substrates containing both types of unsaturated functionalities.
In a recent study Albrecht and co-workers demonstrated that, in
the hydrogenation of enones, the reduction of the olefinic double
bond is kinetically preferred over the reduction of the ketone,35
thus implying that the saturated ketone may be the final product
of the overall process if a suitable catalyst is used. Under these
terms, a suitable catalyst for the selective reduction of the olefin in
these substrates would not able to reduce the final saturated
ketone. Table 5 shows the results of the reduction of benzylidene
ketone and dibenzylidene ketone with glycerol using 1 and 2
(catalyst loading 2.5 mol %) at 120 °C. As can be seen from the
data shown in the table, the reduction of benzylidene ketone is
very selective in the production of the saturated ketone, and only
traces of the fully hydrogenated compound (saturated alcohol)
are detected. For the reduction of dibenzylidene ketone, again
the reduction of the olefinic bonds is more favorable than the
reduction of the ketone, which is never observed. The reduction
of only one of the CdC double bonds is preferred over the
reduction of both olefins; therefore, the major product is always
the monoolefinic ketone.
This result is in contrast with the reduction of cinnamaldehyde
(entries 8 and 9), for which the reduction of the alkenic double
bond is followed by the rapid reduction of the carbonyl group, so
that the major final product is the saturated alcohol. This reaction
also illustrates the higher tendency of the two catalysts 1 and 2 to
reduce aldehydes over ketones.
’ CONCLUSIONS
A new approach for the use of glycerol as solvent and
hydrogen donor has been developed. The introduction of sulfonate
groups attached to the ligand promotes a higher solubility of the
catalysts in glycerol, which combined with the high electron-
donating character of the bis-NHC ligands makes 1 and 2
excellent catalysts for the reduction of several substrates using
glycerol as hydrogen source. Both catalysts proved to be very
active in the reduction of aromatic ketones and aldehydes and
moderately active in the reduction of a selected set of olefins,
acetylenes. Probably the most remarkable result is the selective
reduction of the olefinic double bonds of α,β-unsaturated
ketones. Our results indicate that glycerol may be considered
as a viable hydrogen source for the reduction of a wide variety of
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ARTICLE
organic substrates, if suitable active catalysts are found. Further
research into the preparation of highly active hydrogen transfer
catalysts that may validate the use of glycerol as a suitable
hydrogen source are currently underway in our laboratory.
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’ EXPERIMENTAL SECTION
General Procedures. Compounds 1À4 of general formula
[IrI2(AcO)(bis-NHC)] or [Ru(η6-arene)(NHC)CO3] were prepared
according to literature procedures.21,24,25 All other reagents and solvents
were used as received from commercial suppliers. Catalytic experiments
were carried out under aerobic conditions and without solvent pretreat-
ment. NMR spectra were recorded on Varian Innova 300 and 500 MHz
spectrometers. Elemental analyses were carried out with an EA 1108
CHNS-O Carlo Erba analyzer. Electrospray mass spectra (ESI-MS)
were recorded on a Micromass Quatro LC instrument, and nitrogen was
employed as drying and nebulizing gas. Qualitative analyses of the catalytic
reaction products were performed on a GCMS-QP2010 (Shimadzu) gas
chromatograph/mass spectrometer equipped with a Teknokroma
(TRB-5MS, 30 m  0.25 mm  0.25 μm) column. Chemical yields
were determined with a GC-2010 gas chromatograph (Shimadzu)
equipped with a FID and a Teknokroma (TRB-5MS, 30 m  0.25
mm  0.25 μm) column, using anisole as an internal standard and
analytically pure samples or commercial products for the instrument
calibration.
Catalytic Studies. In a typical experiment of transfer hydrogena-
tion using glycerol as hydrogen donor, a capped vessel containing a
stirrer bar was charged with the substrate (0.5 mmol), potassium
hydroxide (0.5 mmol), anisole as internal reference (0.5 mmol), and
catalyst (2.5%) in 0.8 mL of glycerol. The solution was heated to
80À120 °C for the appropriate time. During the reaction monitoring,
yields and conversions were determined by GC chromatography.
Products and intermediates were characterized by GC/MS. Isolated
products were characterized by 1H NMR and 13C NMR after column
chromatography purification using n-hexanes/ethyl acetate mixtures.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: eperis@qio.uji.es (E.P.), jmata@qio.uji.ex (J.A.M.). Fax:
(+34) 964728214.
(35) Horn, S.; Gandolfi, C.; Albrecht, M. Eur. J. Inorg. Chem.
2011, 2863.
’ ACKNOWLEDGMENT
We thank the Ministerio de Ciencia e Innovaciꢀon of Spain
(CTQ2008-04460) and Bancaixa (P1.1B2010-02 and P1.1B2008-16)
for financial support. We also thank the “Generalitat Valenciana”
for a fellowship (A.A.). We are grateful to the Serveis Centrals
d’Instrumentaciꢀo Científica (SCIC) of the Universitat Jaume I
for providing us with spectroscopic facilities.
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